Motor-driven steering controller and automobile anti-skid controller

ABSTRACT

A motor-driven steering controller for controlling a steering wheel of a vehicle. The controller includes a steering torque controlling unit, a braking force estimating unit, a right and left braking force difference estimating unit and an assist steering torque providing unit. The steering torque controlling unit controls a steering torque on the steering wheel depending on a steering operation. The braking force estimating unit estimates braking forces to be imposed on wheels of the vehicle. The right and left braking force difference estimating unit estimates difference between the braking forces to be imposed on the right and left wheels each estimated by the braking forces estimating unit. The assist steering torque providing unit provides an assist steering torque for the steering torque controlling unit on the basis of the difference in braking force between right and left wheels estimated by the right and left braking force difference estimating unit.

[0001] The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2002-104901 filed Apr. 4, 2002, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a motor-driven steeringcontroller and more particularly to a motor-driven steering controllerwhich provides assist steering torque depending on difference in brakingforce between right and left wheels and an anti-skid controllerincluding such a motor-driven steering controller.

[0004] 2. Background Art

[0005] When braking sudden enough to cause anti-skid control is executedon a road where the frictional coefficient differs from right to left,i.e., so-called μ split road, the resulting braking force differs fromright side to left side of a vehicle. The difference in braking forcebetween the right and left sides causes generation of yaw moment thatcauses the vehicle to be deflected toward the higher road frictionalcoefficient side. On the μ slit surface of an actual road, snow or icecan be left on the edge of the road to reduce the frictional coefficientthereof while the asphalt on the central part of the road is dried orwet to have a raised frictional coefficient. When a vehicle having itsright and left wheels positioned on the edge of the road and on thecentral part of the road, respectively, is suddenly braked, theresulting braking force is larger on the central part of the road, whichhas a large frictional coefficient, than on the edge of the road, whichhas a smaller frictional coefficient. As a result, yaw moment isdeveloped on the vehicle to cause the vehicle to be deflected toward thecentral part of the road.

[0006] In order to reduce yaw moment generated due to the difference inbraking force between right and left, the operator needs to steer in adirection opposite the deflecting direction of the vehicle. Thissteering is known as counter-steering. However, this counter-steeringrequires some deal of skill.

[0007] On the other hand, anti-skid control is normally arranged suchthat the braking force on the front wheels is subjected to yaw momentinhibition control while the braking force on the rear wheels issubjected to select low control to inhibit sudden yaw moment caused bythe difference in braking force between right and left when suddenbraking is executed on μ split road. The yaw moment inhibition controlis adapted to raise the time gradient of braking force on the wheel onhigh frictional coefficient side (i.e., reduce the gradient of rise ofbraking force with respect to time) when the road is judged to be μsplit road, to thereby inhibit the generation of sudden yaw moment. Theselect low control is adapted to control the braking force on highfrictional coefficient side according to the braking force on lowfrictional coefficient side. Both the yaw moment inhibition control andthe select low control reduce the braking force on high frictionalcoefficient side, giving a long braking distance.

[0008] In this respect, JP-A-8-183470 proposes a power steering devicewhich reduces steering torque necessary for steering according to thedifference in braking forces between right and left wheels to facilitatea counter-steering operation. It is specifically described that, whenthe difference in braking forces between the right and left wheels issensed to be large while operating the anti-skid controller, solenoidvalves are controlled to control the hydraulic pressure to be suppliedinto a reaction variable mechanism in control valves so that thesteering reaction to be imposed on the operation of the steering wheelby the reaction variable mechanism is reduced.

[0009] JP-A-11-129,927 proposes the running wheel control structure of amotor-driven steering vehicle provided with a torque steer inhibitioncontrolling unit. In this, a problem is taken into account that when avehicle having a right and left wheel control system on board iscontrolled such that driving forces (or braking forces) which are notequal from right to left are acted on the wheels to be steered (frontwheels in ordinary vehicle), moment developed around the king pin bydriving/braking force differs from the right wheel to the left wheelwhen an ordinary suspension is used, causing a steering kickback, lossof control over a steering wheel due to the turning of the wheels to besteered.

[0010] The above cited official gazette describes an arrangement of thedevice as follows. That is, when there occurs difference in controlvalue between the right and left wheel while the driving/braking forcecontrol is separately executed over the right and left wheels, torquesteer inhibition control signal is outputted to the motor-driven powersteering device to cancel moment (torque steer) developed around theking pin due to the difference in driving force/braking force betweenthe right and left wheels, making it possible to execute torque steerinhibition control in addition to ordinary assist control by themotor-driven power steering device.

SUMMARY OF THE INVENTION

[0011] The power steering device disclosed in the above citedJP-A-8-183470 is adapted to reduce steering torque duringcounter-steering operation, making it easy for an operator who canexecute counter-steering operation to operate the power steering device.However, the power steering device is not necessarily useful for oneswho cannot operate counter-steering properly.

[0012] The device disclosed in JP-A-11-129927 is adapted to executetorque steer inhibition control for canceling moment (torque steer)developed around the king pin due to the difference in drivingforce/braking force between the right and left wheels. The king pin isthe central axis of steering of wheel in the suspension having asteering system. There are a case where the grounding point of the kingpin axis on the grounding surface of the wheel is inside the applicationpoint (positive king pin offset) and a case where the grounding point ofthe king pin axis on the grounding surface of the wheel is outside theapplication point (negative king pin offset) For example, in the casewhere braking force on the left wheel is large and braking force on theright wheel is small, when the king pin offset is positive, moment isacted on the right wheel causing the left wheel to take a left turnwhile moment is acted on the right wheel causing the right wheel to takea right turn. During this process, since braking force on the left wheelis larger than on the right wheel, torque is developed causing the wheelto take a left turn. This torque causes a steering kickback (phenomenonthat the wheel causes the steering wheel to rotate counterclockwise). Incontrast, in the case of negative king pin offset, a moment acts on theleft wheel causing the wheel to take a right turn while moment acts onthe right wheel causing the wheel to take a right turn. During thisprocess, the difference in braking force between the right and leftwheels causes the development of moment causing the wheels to takearight turn. This torque causes loss of control over the steering wheel.

[0013] Thus, regardless of whether the king pin offset is positive ornegative, the difference in braking force between the right and leftwheels makes moments around the king pin unbalanced, causing thesteering kickback i.e., torque steering. The device disclosed in theabove JP-A-11-129927 is aimed to inhibit this torque steering. In otherwords, the aforementioned device is aimed to prevent the steeringkickback due to the difference in braking force between the right andleft wheels. It is arranged such that when there occurs difference inbraking force between the right and left wheels during forward running,auxiliary torque corresponding to the difference in braking forcebetween the right and left wheels is added to prevent the steeringkickback, that is, to keep the steering wheel straight. It is certainthat the negative king pin offset due to mechanical and geometricalconfiguration of the suspension can steer the wheels and steering wheelin the direction opposite to the direction in which the vehicle isdeflected by the difference in braking force between the right and leftwheels, by making the use of unbalance of torque around the king pinshaft. However, a complicated structure is needed to realize a negativeking pin offset by the configuration of suspension.

[0014] The invention relates to a motor-driven steering controller andits an aim is to facilitate counter-steering operation for inhibitingthe deflection of a vehicle toward high frictional coefficient side dueto the difference in braking force between the right and left wheelsoccurring when braking force is provided to the wheels on so-called μsplit road.

[0015] Another aim of the invention is to facilitate counter-steeringoperation during anti-skid control in the anti-skid controller of avehicle provided with the aforementioned motor-driven steeringcontroller, thereby making the effective use of braking force on highfrictional coefficient side to reduce the braking distance.

[0016] In order to solve the aforementioned problems, the inventionprovides a motor-driven steering controller for controlling a steeringwheel of a vehicle, including: a steering torque controlling unit forcontrolling a steering torque on the steering wheel depending on asteering operation of the vehicle; a braking force estimating unit forestimating braking forces to be imposed on wheels of the vehicle; aright and left braking force difference estimating unit for estimatingdifference between the braking forces to be imposed on the right andleft wheels each estimated by the braking forces estimating unit; and anassist steering torque providing unit for providing an assist steeringtorque for the steering torque controlling unit on the basis of thedifference in braking force between right and left wheels estimated bythe right and left braking force difference estimating unit.

[0017] Preferably, the motor-driven steering controller further includesa vehicle stability controlling unit for controlling a stability of thevehicle depending on a traveling state of the vehicle. The assiststeering torque providing unit is arranged so as to inhibit provision ofthe assist steering torque when the stability of the vehicle iscontrolled by the vehicle stability controlling unit.

[0018] Preferably, the assist steering torque providing unit is arrangedso as to set the assist steering torque depending on a temporalvariation in the difference between the braking forces of the right andleft wheels estimated by the right and left braking force differenceestimating unit.

[0019] Preferably, the assist steering torque providing unit is arrangedso as to set the assist steering torque larger as a velocity of thevehicle increases.

[0020] Preferably, the assist steering torque providing unit is arrangedso as to set the assist steering torque larger when the vehicle takes aturn than when the vehicle goes straight ahead.

[0021] The invention further provides an anti-skid controller in avehicle including a motor-driven steering controller for controlling asteering wheel of the vehicle, wherein the motor-driven steeringcontroller includes a steering torque controlling unit for controlling asteering torque on the steering wheel depending on a steering operationof the vehicle, a braking force estimating unit for estimating brakingforces to be imposed on wheels of the vehicle, a right and left brakingforce difference estimating unit for estimating difference between thebraking forces to be imposed on the right and left wheels each estimatedby the braking forces estimating unit, and an assist steering torqueproviding unit for providing an assist steering torque for the steeringtorque controlling unit on the basis of the difference in braking forcebetween right and left wheels estimated by the right and left brakingforce difference estimating unit. The anti-skid controller includes acontrol parameter setting unit for setting a first control parameterwhen provision of the assist steering torque by the assist steeringtorque providing unit is not effected and for setting a second controlparameter different from the first control parameter when provision ofthe assist steering torque by the assist steering torque providing unitis effected; and a braking force controlling unit for controlling thebraking forces to be imposed on the wheels depending on the firstcontrol parameter set by the control parameter setting unit and thesecond control parameter set by the control parameter setting unit.

[0022] Preferably, the control parameter setting unit is arranged so asto set the first control parameter to be a parameter for reductioncontrol of yaw moment that causes a braking force for one of frontwheels of the vehicle to rise slowly with time gradient when anti-skidcontrol begins on the other front wheel. The control parameter settingunit is arranged so as to set the second control parameter to be aparameter having a larger time gradient than the first control parameteror a parameter for inhibiting the reduction control of yaw moment.

[0023] Preferably, the control parameter setting unit is arranged so asto set the first control parameter to be a select low control parameterfor controlling a braking force on one of rear wheels of the vehicle inthe same manner as on the other rear wheel when anti-skid begins on theother rear wheel. Further, the control parameter setting unit isarranged so as to set the second control parameter to be a parameter forcausing the braking force on the one of the rear wheels to rise withtime gradient after a predetermined period of time of select lowcontrol.

[0024] Preferably, the control parameter setting unit is arranged so asto set the second control parameter when the turning condition of thevehicle is not beyond a predetermined value.

[0025] Preferably, the anti-skid controller further includes a steeringdirection judging unit for judging a direction of steering by thesteering operation. The control parameter setting unit is arranged so asto inhibit the setting of the second control parameter when the steeringdirection judging unit judges that steering is not made in the directionfor a counter-steering operation regardless of the fact that a provisionof assist steering torque by the assist steering torque providing unitis effected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention may be more readily described withreference to the accompanying drawings, in which:

[0027]FIG. 1 is a diagram illustrating the outline of a motor-drivensteering controller according to an embodiment of implementation of theinvention;

[0028]FIG. 2 is a block diagram illustrating the system configurationaccording to the embodiment of implementation of the invention;

[0029]FIG. 3 is a control block diagram of the motor-driven steeringcontroller according to the embodiment of implementation of theinvention;

[0030]FIG. 4 is a schematic diagram illustrating an anti-skidcontrolling system according to the embodiment of implementation of theinvention;

[0031]FIG. 5 is a flow chart illustrating the processing ofcounter-steering assist control in the embodiment of implementation ofthe invention;

[0032]FIG. 6 is a flow chart illustrating the operation ofcounter-steering assist electric current instruction in the embodimentof implementation of the invention;

[0033]FIG. 7 is a graph illustrating the characteristics ofcounter-steering assist steering torque in the embodiment ofimplementation of the invention according to the difference in brakingforce between the right and left wheels;

[0034]FIG. 8 is a graph illustrating the characteristics ofcounter-steering assist steering torque in the embodiment ofimplementation of the invention according to the vehicle velocity;

[0035]FIG. 9 is a flow chart illustrating the processing of anti-skidcontrol in the case of execution of counter-steering assist control inthe embodiment of implementation of the invention;

[0036]FIG. 10 is a graph illustrating the characteristics of brake fluidpressure in specific anti-skid control over the front wheels duringcounter-steering assist control in the embodiment of implementation ofthe invention;

[0037]FIG. 11 is a graph illustrating the characteristics of brake fluidpressure in specific anti-skid control over the rear wheels duringcounter-steering assist control in the embodiment of implementation ofthe invention; and

[0038]FIG. 12 is a flow chart illustrating the processing ofcounter-steering assist control in the embodiment of implementation ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Embodiment of implementation of the invention will be describedhereinafter in connection with the attached drawings. FIG. 1 illustratesthe outline of a motor-driven steering controller (also referred to as“motor-driven power steering controller”) according to an embodiment ofimplementation of the invention. The motor-driven steering controller isarranged as follows: Steering torque acting on a steering shaft 2according to the operator's operation on a steering wheel 1 is detectedby a steering torque sensor TS. Steering angle is detected by arotational angle sensor RS described later. An EPS motor (electricmotor) 3 is drive-controlled on the basis of these detection signals andvehicle velocity signal (represented by VS), to thereby steer the frontwheels via a reduction gear 4 and a rack-and-pinion 5, reducing theoperator's power required to operate the steering wheel 1. Inparticular, the motor-driven steering controller is arranged so as toprovide steering torque according to the difference in braking forcebetween right and left wheels, assisting the operation duringcounter-steering. The control over the provision of assist steeringtorque according to the difference in braking force between right andleft wheels will be hereinafter referred to as “counter-steering assistcontrol”.

[0040] The motor-driven steering controlling unit ECU1 according to thepresent embodiment is provided with a steering torque control block B0for controlling steering torque on the steering wheel of a vehicleaccording to steering operation. The motor-driven steering controllingunit ECU1 is also provided with a braking force estimating block B1 forestimating braking force to be imposed on the various wheels of thevehicle, a right and left braking force difference estimating block B2for estimating the difference between the braking force to be imposed onthe right and left wheels on the basis of the braking force estimated bythe braking force estimating block B1 and an assist steering torqueproviding block B3 for providing an assist steering torque for thesteering torque controlling block B0 on the basis of the difference inbraking force between right and left wheels estimated by the right andleft braking force difference estimating block B2. The difference inbraking force between right and left wheels is the difference betweenbraking force imposed on the left wheel of the vehicle and braking forceimposed on the right wheel. As the former braking force there is usedthe sum of braking force imposed on the left front and rear wheels orbraking force imposed on the left front wheel. As the former brakingforce there is used the sum of braking force imposed on the right frontand rear wheels or braking force imposed on the right front wheel.

[0041] In the braking force estimating block B1, signals from fluidpressure sensors (typically represented by PS) and wheel velocitysensors (typically represented by WS) provided on the wheels are used toestimate braking force developed on the wheels according to an equationof motion of wheel. In the right and left braking force differenceestimating block B2, the difference in braking force between right andleft wheels is calculated. Since a specific unit concerning theestimation of braking force is described in JP-A-2000-108863, a detaileddescription for that is omitted herein. Further, in the assist steeringtorque providing block B3, assist steering torque is determined to beprovided according to the difference in braking force between right andleft wheels. Corresponding to this torque, electric current instructionis then determined. Electric current instruction required to assistcounter-steering (counter-steering assist electric current instruction)is then added to electric current instruction determined by an ordinarymotor-driven steering controller (EPS) (hereinafter referred to as “EPSelectric current instruction”) to control EPS motor 3. In the presentembodiment, an anti-skid controlling unit ECU2 is further provided. Theanti-skid controlling unit ECU2 is connected to the aforementionedmotor-driven steering controlling unit ECUL in the following manner.

[0042]FIG. 2 illustrates the system configuration of the presentembodiment. The motor-driven steering controlling system and theanti-skid controlling system are connected to each other via acommunication bus so that mutual system data can be shared between thetwo systems. In the motor-driven steering controlling system, to themotor-driven steering controlling unit ECU1 including a CPU, a ROM and aRAM for motor-driven steering control is connected to a steering torquesensor TS and a rotational angle sensor RS and to an EPS motor 3 via amotor driving circuit MA. In the anti-skid controlling system, on theother hand, to an anti-skid controlling unit ECU2 including a CPU, a ROMand a RAM for anti-skid control is connected to a wheel velocity sensorWS, a fluid pressure PS and a stop switch ST, and to solenoid valves PC1to PC8 via a solenoid driving circuit SA. The motor-driven steeringcontrolling unit ECU1 and the anti-skid controlling unit ECU2 are eachconnected to the communication bus via a communication unit including aCPU, a ROM and a RAM for communication. Thus, in the anti-skidcontrolling system, data on braking force developed in the variouswheels are obtained, and the difference in braking force between rightand left wheels is calculated on the basis of the data. In themotor-driven steering controlling system, counter-steering assistcontrol is effected. Data on vehicle velocity required for themotor-driven steering control also can be transmitted from the anti-skidcontrolling system.

[0043]FIG. 3 is a control block diagram of the motor-driven steeringcontroller. In the blocks A1 to A4, an assist control, a torque inertiacompensation control, a steering return control and a dampercompensation control are executed on the basis of a steering torque anda vehicle velocity signal to determine an electric current instructionvalue for driving the EPS motor 3. Referring to the assist control inthe block A1, a torque assist for reducing the operator's power requiredto operate the steering wheel 1 is effected. Referring to the torqueinertia compensation control in the block A2, a control is effected tocompensate response delay due to inertia of the EFS motor 3. Referringto steering return control in the block A3, a control is effected toimprove return of the steering wheel 1 to the neutral point. Referringto damper compensation control in the block A4, control is effected toinhibit overreturn of the steering wheel 1 and improve the convergenceof the steering wheel 1. In the case where as the EPS motor 3 there isused a brushless DC motor as in the present embodiment, the EPS motor 3is provided with a rotational angle sensor RS by which steering angle isdetermined for EPS control and operation in the block A0. For example,the rack and pinion 5 may be provided with a steering angle sensor togive a detection signal on the basis of which steering angle is thendetermined.

[0044]FIG. 4 is a diagram illustrating a brake system including ananti-skid controlling system. The anti-skid controlling system includessolenoid valves (PC1 to PC8), fluid pressure pump HP1 and HP2, a motorM, reservoirs RS1 and RS2, wheel velocity sensors (WS1 to WS4), andfluid pressure sensors (PS1 to PS6) In FIG. 4, the solenoid valves PC1to PC8, etc. are provided interposed between a master cylinder MC andwheel cylinders Wfr, Wfl, Wrr and Wrl. In other words, on the fluidpressure paths connecting between one of the output ports of the mastercylinder MC and the wheel cylinders Wfr and Wrf, respectively, areprovided normally-open solenoid valves PC1 and PC7, respectively. To themiddle point on the path between the solenoid valves PC1 and PC7 and themaster cylinder MC is connected the discharge side of the fluid pressurepump HP1. Similarly, on the fluid pressure paths connecting between theother output port of the master cylinder MC and the wheel cylinders Wfland Wrr, respectively, are provided normally-open solenoid valves PC3and PC5, respectively. To the middle point on the path between thesolenoid valves PC3 and PC5 and the master cylinder MC is connected thedischarge side of the fluid pressure pump HP2. The fluid pressure pumpsHP1 and HP2 are each driven by the electric motor M. When these pumpsare operated, a brake fluid the pressure of which has been raised to apredetermined value is supplied into the aforementioned fluid pressurepaths.

[0045] The wheel cylinders Wfr and Wfl are connected to normally-opensolenoid valves PC2 and PC8, respectively. These solenoid valves eachhave a reservoir RS1 provided downstream and are connected to thesuction side of the fluid pressure pump H1. Similarly, the wheelcylinders Wfl and Wrr are connected to normally-open solenoid valves PC4and PC6, respectively. These solenoid valves each have a reservoir RS2provided downstream and are connected to the suction side of the fluidpressure pump H2. The reservoirs RS1 and RS2 each have a piston and aspring and receive the brake fluid for the various wheel cylindersdischarged via the solenoid valves PC2 and PC4 and the solenoid valvesPC6 and PC8, respectively.

[0046] The solenoid valves PC1 to PC8 each are a two-port andtwo-position electromagnetic switching valve. When the solenoid coil ofthese valves are not energized, these valves are in first position shownin FIG. 4 so that the wheel cylinders Wfr, Wfl, Wrr and Wrl arecommunicated to the master cylinder MC. When the solenoid coil of thesevalves are energized, these valves are in second position so that thewheel cylinders Wfr, Wfl, Wrr and Wrl are disconnected from the mastercylinder MC but communicated to the reservoir RS1 or R52. A check valveCV is adapted to allow the reflux from the wheel cylinders Wfr, Wfl, Wrrand Wrl to the master cylinder MC but shut down the reverse flow. InFIG. 4, OR indicates an orifice, D indicates a damper, HB indicates abooster, and BP indicates a brake pedal. ST indicates a stop switchwhich is ON when the brake pedal is operated.

[0047] By controlling such that the solenoid coil of these solenoidvalves PC1 to PC8 are energized or deenergized, the pressure of thebrake fluid in the wheel cylinders Wfr, Wfl, Wrr and Wrl can be raised,reduced or kept. In other words, when the solenoid coil of the solenoidvalves PC1 to PC8 are not energized, a brake fluid is supplied into thewheel cylinders Wfr, Wfl, Wrr and Wrl from the master cylinder MC andthe fluid pressure pump HP1 or HP2 to raise the pressure in these wheelcylinders. When the solenoid coil of the solenoid valves PC1 to PC8 areenergized, the wheel cylinders Wfr, Wfl, Wrr and Wrl are communicated tothe reservoir RS1 or RS2 to reduce their pressure. When the solenoidvalve of the solenoid valves PC1, PC3, PC5 and PC7 are energized but thesolenoid coil of the other solenoid valves are not energized, the brakefluid pressure in the wheel cylinders wfr, Wfl, Wrr and Wrl are kept.Accordingly, by adjusting the duty ratio according to the condition ofthe wheels and repeating the energization/deenergization of theaforementioned solenoid coils according to the duty ratio, fluidpressure control in pulse pressure increase mode (also referred to as“step pressure increase mode”) can be effected to slowly increase thepressure. In pulse pressure increase mode, control can be effected toslowly reduce the pressure.

[0048] The aforementioned solenoid valves PC1 to PC8 are connected tothe anti-skid controlling unit ECU2 so that their solenoid coils arecontrolled to be energized or deenergized. The electric motor M, too, isconnected to the anti-skid unit ECU2 so that it is controlled to bedriven. The fluid pressure in the wheel cylinders may be controlled bylinear valves (not shown) instead of the solenoid valves PC1 to PC8. Inorder to generate braking force, a motor or the like may be used toprovide a mechanical braking force (not shown) without using brake fluidpressure.

[0049]FIG. 5 is a flow chart illustrating the processing ofcounter-steering assist control in the aforementioned configuration ofmotor-driven steering controller. Firstly, initialization is executed atstep 101. At step 102, sensor signal is inputted. At step 103, necessarysignal from the communication bus is read. At step 104, filtering andvarious signal processings necessary for control are executed. At step105, the EPS electric current instruction value necessary for ordinarymotor-driven steering control is determined on the basis of thesesignals.

[0050] Subsequently, at step 106, it is judged to see ifcounter-steering assist control has been already executed. Ifcounter-steering assist control has been already executed, the processproceeds to step 107 where control termination is then judged. Ifcounter-steering assist control is not executed, the process proceeds tostep 108 where control start is judged. At step 108, it is judged to seeif the conditions are satisfied that the vehicle is being braked (stopswitch ST is kept ON), the difference in braking force between rightwheel and left wheel is not smaller than a predetermined value and thevehicle velocity is not smaller than a predetermined value. If theconditions are satisfied, the process proceeds to step 109 wherecounter-steering assist control is executed.

[0051] At step 107, control termination is judged. That is, if any ofthe conditions that the vehicle is not being braked (stop switch ST iskept OFF), the difference in braking force between the right wheel andthe left wheel falls below a predetermined value and the vehiclevelocity falls below a predetermined value is satisfied,counter-steering assist control is terminated. The process then proceedsto step 110. If counter-steering assist control is initiated orcontinues, the process proceeds to step 109 where counter-steeringassist electric current instruction is calculated. At step 110, thisinstruction is added to EPS instruction to control EPS motor 3.

[0052]FIG. 6 illustrates the operation of counter-steering assistelectric instruction executed at step 109. Firstly, at step 201, thedirection in which counter-steering is to be effected is judged on thebasis of braking force on the various wheels detected by the anti-skidcontrolling system. If the braking force on the right wheel is largerthan the braking force on the left wheel, it is judged thatcounter-steering in the right turning direction is necessary. Incontrast, if the braking force on the left wheel is larger than thebraking force on the right wheel, it is judged that counter-steering inthe left turning direction is necessary. Subsequently, at step 202, thedifference in braking force between right wheel and left wheel iscalculated. Further, at step 203, the change of the difference inbraking force between right wheel and left wheel with time iscalculated. At step 204, the steering torque necessary forcounter-steering assist is calculated on the basis of these results ofcalculation. At step 205, the value for counter-steering assist electriccurrent instruction is calculated on the basis of the results ofcalculation.

[0053] The counter-steering assist steering torque (hereinafter simplyreferred to as “assist steering torque”) calculated at step 204 is setto be larger as the difference in braking force between right and leftwheels increases and the rate of temporal variation of the difference inbraking force between right and left wheels increases as shown in FIG.7. Since the assist steering torque is set to have an upper limit of Tauas shown in FIG. 7, the operator can override steering operation even iferroneous calculation is executed. When the vehicle velocity is large,the variation of yaw moment due to the difference in braking forcebetween right and left wheels is large, making it difficult for theoperator to cope with these difficulties. It is therefore preferablyarranged such that assist steering torque increases with the rise ofvehicle velocity as shown in FIG. 8.

[0054] The process thus proceeds to step 205 where correspondingelectric current instruction value is determined according to thesteering torque necessary for counter-steering assist. Thiscounter-steering electric current instruction value is then added to EPSelectric current instruction value to control the EPS motor 3. Sincesteering torque is added in the direction of counter-steering, thesteering wheel is operated in the counter-steering direction accordingto the difference in braking force between right and left wheels.Accordingly, an operator who cannot execute proper counter-steering caneasily execute counter-steering.

[0055] As mentioned above, the counter-steering assist control by theprovision of steering torque makes it possible to secure the directionalstability of the vehicle during braking on μ split road. Thus, anti-skidcontrol can be properly executed, making it possible to improve brakingperformance of the vehicle. An ordinary anti-skid control is executedsuch that yaw moment is inhibited on the front wheels and select lowcontrol is executed on the rear wheels for the purpose of securing thevehicle stability during anti-skid control on μ split road. Both thecontrols are adapted to inhibit the braking force on high μ side wherebraking force can be afforded and reduce yaw moment that induces vehicleinstability. The counter-steering assist control can be executed tosecure vehicle stability by steering. By properly setting anti-skidcontrol parameters, both vehicle stability and braking performance canbe secured.

[0056]FIG. 9 illustrates the processing of anti-skid control during theexecution of counter-steering assist control. Firstly, at step 301,initialization is executed. At step 302, a sensor signal is inputted. Atstep 303, a communication signal from the communication bus is read. Thecommunication signal is a steering angle signal to be provided forcontrol flag of counter-steering assist control and motor-drivensteering control. Subsequently, at step 304, filtering and varioussignal processings necessary for control are executed. At step 305,control parameter necessary for ordinary anti-skid control (i.e., firstcontrol parameter) is set.

[0057] Subsequently, at step 306, it is judged to see ifcounter-steering assist control is being executed. If it is judged thatcounter-steering assist control is being executed, the process proceedsto step 307 where a specific parameter (i.e., second control parameter)is then set. The process then proceeds to step 308. If it is judged thatcounter-steering assist control is not being executed, the processproceeds to step 308 with the first control parameter set at step 305.At step 308, anti-skid control is then executed. If counter-steeringassist control is executed, the directional stability of the vehicle issecured by steering. Accordingly, the second control parameter ispredetermined to make the effective use of braking force on the sidehaving a high road frictional coefficient to reduce the brakingdistance. The specific control parameter on the front and rear wheels(second control parameter) will be described hereinafter.

[0058]FIG. 10 illustrates specific anti-skid control over the frontwheels during counter-steering assist control. When the operator works abrake pedal BP at time TO, the master cylinder fluid pressure rises from0 to Pm as shown by the broken line. When the front wheel on low μ sidereaches its frictional limit, e.g., at time T1, anti-skid control isthen executed. At this point, the front wheel on high μ side issubjected to yaw moment inhibition control so that the master cylinderfluid pressure rises slowly from Pfl to Pfh as shown by the one-dottedchain line in FIG. 10 with such a time gradient that no unnecessary yawmoment is generated. In contrast, if counter-steering inhibition controlis executed, control is executed such that the fluid pressure rises fromPfl to Pfh as shown by the two-dotted chain line in FIG. 10 with alargely-set time gradient because yaw moment inhibition is executed bysteering. Further, yaw moment inhibition control may be inhibited. Inthis case, the brake fluid pressure on high μ side wheel rises similarlyto the master cylinder fluid pressure.

[0059]FIG. 11 illustrates specific anti-skid control over the rearwheels during counter-steering assist control. In general, in select lowcontrol, if anti-skid control begins on low μ side wheel at time T1,high μ side wheel is subjected to the same anti-skid control as low μside wheel is Thus, the brake fluid pressure on high μ side wheel islimited in the same manner as low μ sidewheel is. In contrast, ifcounter-steering assist control is executed, select low control isexecuted for only a predetermined period of time (between T1 and T2 inFIG. 11) to inhibit sudden yaw change in the initial stage of braking.Thereafter, control is executed such that the fluid pressure graduallyrises with a time gradient as shown by the two-dotted chain line. If thevehicle velocity is large, the period for executing the select lowcontrol is set to be long, and/or the time gradient of rise of brakefluid pressure on high μ side is set to below, in order to furthersecure the vehicle stability.

[0060] It is further necessary to take into account the situation inwhich, if counter-steering assist control is executed, the operator holdthe steering wheel 1. FIG. 12 illustrates the processing to be executedif at step 307 in FIG. 9, specific anti-skid control is executed duringcounter-steering assist control as shown in FIGS. 10 and 11, i.e., onthe basis of the second control parameter. Firstly, at step 401, theoperator's steering operation during counter-steering assist control isdetected on the basis of steering angle signal. At step 402, the turningcondition of the vehicle is judged. The process then proceeds to steps403 and 404. If the operator does not execute counter-steering operationeven during counter-steering assist control, or if the turning conditionof the vehicle is beyond a predetermined value, the process then returnsto the routine in FIG. 9 where ordinary anti-skid control (first controlparameter) is then executed. In contrast, if the operator executescounter-steering operation at steps 403 and 404 and if the turningcondition of the vehicle is judged to be not beyond a predeterminedvalue, the process proceeds to steps 405 and 406 where the same specificcontrol parameter (second control parameter) as mentioned above is thenset.

[0061] Since the turning of the vehicle is accompanied by the shift ofload between the right and left wheels, there occurs a difference inbraking force between the right and left wheels during anti-skid controleven if there is no difference in road frictional coefficient betweenthe right and left wheels. Therefore, the present embodiment is arrangedsuch that if load shift due to turning makes difference in braking forcebetween the right and left wheels, counter-steering assist control isnot executed. Which the difference in braking force is attributed toload shift or difference in road frictional coefficient can be judged bythe turning condition and the brake fluid pressure. The turningcondition of the vehicle can be calculated from the steering angle andvehicle velocity. If there are provided a yaw rate sensor and atransverse acceleration sensor, these sensor signals may be utilized.Therefore, load shift can be determined from the turning condition. Itcan be judged from the fluid pressure sensor signal to see whichanti-skid control has been executed due to load shift or reduction ofroad frictional coefficient. In other words, it is judged to see whichthe difference in braking force between the right and left wheels duringturning has occurred due to load shift alone or difference inroadfrictional coefficient between the right and left wheels.

[0062] The difference in braking force between the right and left wheelsdue to load shift occurs when the turning condition of the vehicleincreases (when the transverse acceleration increases). However, whenthe difference in braking force between the right and left wheels occursdue to difference in road frictional coefficient between the right andleft wheels, the turning condition of the vehicle doesn't increase (thevehicle cannot take a turn with excessive transverse acceleration)because one of the wheels is on low μ side. Accordingly, a simplearrangement may be made such that if the turning of the vehicle occurswith a transverse acceleration of not lower than a predetermined value,it is judged that some difference in braking force between the right andleft wheels due to load shift has occurred and counter-steering assistcontrol is not executed.

[0063] In the case where the vehicle is taking a turn and there occurs adifference in road μ between the right and left wheels (turning μsplit), if the outer wheel on the turning curve is positioned on low μroad, braking causes the drop of side force and causes the vehicle toshift outwardly from the turning curve. Further, the difference inbraking force between the right and left wheels makes it easy for boththe rear wheels to ride on low μ road side, occasionally rendering thevehicle extremely unstable. When the inner wheel on the turning curve ispositioned on low μ road, both side force drop and difference in brakingforce between the right and left wheels act to cause the vehicle toshift outwardly from the turning curve. Therefore, in the case ofturning μ split, it is preferable that the amount of counter-steeringassist control be predetermined to be larger than during forwardrunning. In this case, the controlled amount of steering torque to beprovided is preferably predetermined according to the turning condition.

[0064] While the present embodiment has been described with reference tothe case where the difference in braking force between the right andleft wheels is detected by the fluid pressure sensor PW provided on thewheel cylinder of the wheels, the invention is not limited thereto. Forexample, the brake fluid pressure on the various wheels may bedetermined from the detection signals from the fluid pressure sensorsPS1 and PS2 of the master cylinder MC and the driven state of thesolenoid valves PC1 to PC8 to estimate the difference in braking forcebetween the right and left wheels.

[0065] A vehicle provided with a vehicle stability controlling unit suchas vehicle having a motion controller disclosed in JP-A-9-301148intentionally controls the difference in braking force between the rightand left wheels to generate yaw moment for stabilizing the vehicleduring vehicle stability control. However, since counter-steering assistcontrol acts to inhibit yaw moment caused by the difference in brakingforce between the right and left wheels, the two controls have opposingeffects. Accordingly, if vehicle stability control is executed, it ispreferred that counter-steering control be not executed. The presentembodiment is arranged such that if vehicle stability control by avehicle stability controlling unit is executed, the provision of assiststeering torque is inhibited.

[0066] Being arranged as mentioned above, the invention has thefollowing advantages. In some detail, steering torque is added in thecounter-steering direction by an assist steering torque providing unit,making it assured that the steering wheel can be operated in thecounter-steering direction according to the difference in braking forcebetween the right and left wheels. Accordingly, even an operator whocannot operate proper counter-steering can easily executedcounter-steering.

[0067] In the case where a vehicle stability controlling unit isprovided, counter-steering assist control is not executed, making itpossible to control vehicle stability properly.

[0068] The aforementioned assist steering torque providing unit executesproper counter-steering according to the operational condition of thevehicle.

[0069] Referring to the anti-skid controller of the vehicle providedwith a motor-driven steering controller, counter-steering can be easilyexecuted during anti-skid control and the braking force on highfrictional coefficient side can be made the best use of, making itpossible to reduce the braking distance.

[0070] Further, by using the control parameter setting unit, anti-skidcontrol can be properly executed according to the operational state ofthe vehicle.

[0071] Further, by using the system having the steering directionjudging unit and the aforementioned control parameter setting unit,anti-skid control can be properly executed according to the turningcondition of the vehicle.

What is claimed is:
 1. A motor-driven steering controller forcontrolling a steering wheel of a vehicle, comprising: a steering torquecontrolling unit for controlling a steering torque on the steering wheeldepending on a steering operation of the vehicle; a braking forceestimating unit for estimating braking forces to be imposed on wheels ofthe vehicle; a right and left braking force difference estimating unitfor estimating difference between the braking forces to be imposed onthe right and left wheels each estimated by the braking forcesestimating unit; and an assist steering torque providing unit forproviding an assist steering torque for the steering torque controllingunit on the basis of the difference in braking force between right andleft wheels estimated by the right and left braking force differenceestimating unit.
 2. The motor-driven steering controller as claimed inclaim 1, further comprising: a vehicle stability controlling unit forcontrolling a stability of the vehicle depending on a traveling state ofthe vehicle; wherein the assist steering torque providing unit isarranged so as to inhibit provision of the assist steering torque whenthe stability of the vehicle is controlled by the vehicle stabilitycontrolling unit.
 3. The motor-driven steering controller as claimed inclaim 1, wherein the assist steering torque providing unit is arrangedso as to set the assist steering torque depending on a temporalvariation in the difference between the braking forces of the right andleft wheels estimated by the right and left braking force differenceestimating unit.
 4. The motor-driven steering controller as claimed inclaim 1, wherein the assist steering torque providing unit is arrangedso as to set the assist steering torque larger as a velocity of thevehicle increases.
 5. The motor-driven steering controller as claimed inclaim 1, wherein the assist steering torque providing unit is arrangedso as to set the assist steering torque larger when the vehicle takes aturn than when the vehicle goes straight ahead.
 6. An anti-skidcontroller in a vehicle including a motor-driven steering controller forcontrolling a steering wheel of the vehicle, wherein the motor-drivensteering controller includes a steering torque controlling unit forcontrolling a steering torque on the steering wheel depending on asteering operation of the vehicle, a braking force estimating unit forestimating braking forces to be imposed on wheels of the vehicle, aright and left braking force difference estimating unit for estimatingdifference between the braking forces to be imposed on the right andleft wheels each estimated by the braking forces estimating unit, and anassist steering torque providing unit for providing an assist steeringtorque for the steering torque controlling unit on the basis of thedifference in braking force between right and left wheels estimated bythe right and left braking force difference estimating unit; theanti-skid controller comprising: a control parameter setting unit forsetting a first control parameter when provision of the assist steeringtorque by the assist steering torque providing unit is not effected andfor setting a second control parameter different from the first controlparameter when provision of the assist steering torque by the assiststeering torque providing unit is effected; and a braking forcecontrolling unit for controlling the braking forces to be imposed on thewheels depending on the first control parameter set by the controlparameter setting unit and the second control parameter set by thecontrol parameter setting unit.
 7. The anti-skid controller as claimedin claim 6, wherein the control parameter setting unit is arranged so asto set the first control parameter to be a parameter for reductioncontrol of yaw moment that causes a braking force for one of frontwheels of the vehicle to rise slowly with time gradient when anti-skidcontrol begins on the other front wheel; and the control parametersetting unit is arranged so as to set the second control parameter to bea parameter having a larger time gradient than the first controlparameter or a parameter for inhibiting the reduction control of yawmoment.
 8. The anti-skid controller as claimed in claim 6, wherein thecontrol parameter setting unit is arranged so as to set the firstcontrol parameter to be a select low control parameter for controlling abraking force on one of rear wheels of the vehicle in the same manner ason the other rear wheel when anti-skid begins on the other rear wheel;and the control parameter setting unit is arranged so as to set thesecond control parameter to be a parameter for causing the braking forceon the one of the rear wheels to rise with time gradient after apredetermined period of time of select low control.
 9. The anti-skidcontroller as claimed in claim 7, wherein the control parameter settingunit is arranged so as to set the second control parameter when theturning condition of the vehicle is not beyond a predetermined value.10. The anti-skid controller as claimed in claim 8, wherein the controlparameter setting unit is arranged so as to set the second controlparameter when the turning condition of the vehicle is not beyond apredetermined value.
 11. The anti-skid controller as claimed in claim 6,further comprising a steering direction judging unit for judging adirection of steering by the steering operation; wherein the controlparameter setting unit is arranged so as to inhibit the setting of thesecond control parameter when the steering direction judging unit judgesthat steering is not made in the direction for a counter-steeringoperation regardless of the fact that a provision of assist steeringtorque by the assist steering torque providing unit is effected.